Near-to-Eye and See-Through Holographic Displays

ABSTRACT

A holographic display is comprised of space-multiplexed elemental modulators, each of which consists of a surface acoustic wave transducer atop an anisotropic waveguide. Each “line” of the overall display consists of a single anisotropic waveguide across the display&#39;s length with multiple surface acoustic wave transducers spaced along the waveguide length, although for larger displays, the waveguide may be divided into segments, each provided with separate illumination. Light that is undiffracted by a specific transducer is available for diffraction by subsequent transducers. Per transducer, guided-mode light is mode-converted to leaky-mode light, which propagates into the substrate away from the viewer before encountering a volume reflection grating and being reflected and steered towards the viewer. The display is transparent and all reflection volume gratings operate in the Bragg regime, thereby creating no dispersion of ambient light.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/365,973, filed Jul. 22, 2016, the entire disclosure of which isherein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government Support under Grant NumberFA8650-14-C-6571, awarded by the Air Force Research Laboratory. TheGovernment has certain rights in this invention.

FIELD OF THE TECHNOLOGY

The present invention relates to holographic video displays and, inparticular, to a transparent flat-panel holographic video displaysuitable for near-to-eye and see-through augmented reality applications.

BACKGROUND

The limitations and affordances of holographic video displays arechiefly dictated by the spatial light modulators upon which they arebuilt. The temporal bandwidth of the spatial light modulator determinesthe display size, view angle, and frame rate. The pixel pitch determinesthe angle of the display or the power of the lenses needed to achieve awide view angle. The space-bandwidth product, which is related to thenumerical aperture of the holographic grating, determines the maximumdepth range and number of resolvable views the display will possess.Finally, optical non-idealities of the modulator give rise to noise andartifacts in the display output.

Current state-of-the-art technologies for spatial light modulation, suchas liquid crystal (LC), micro-electro-mechanical systems (MEMS) [Kreis,T., Aswendt, P., and Hofling, R., “Hologram reconstruction using adigital micromirror device,” Optical Engineering, vol. 40, pp. 926-933(2001); Pearson, E., “MEMS spatial light modulator for holographicdisplays”, S.M. Thesis, Massachusetts Institute of Technology (2001)],and bulk-wave acousto-optic modulators [Hilaire, P., Benton, S., andLucente, M., “Synthetic aperture holography: a novel approach tothree-dimensional displays,” Journal of the Optical Society of AmericaA, vol. 9, pp. 1969-1977 (1992)], have proven challenging to employ inholographic video displays. The modulators employed currently arechallenging to use for several reasons: low bandwidth (relative toholograms), high cost, low diffraction angle, poor scalability,quantization error, and the presence of noise, unwanted diffractiveorders, and zero-order light, as well as the requirement for spatial ortemporal multiplexing of color. These issues must therefore be addressedbefore using these modulators in a holographic display system.

Much of the cost and complexity of modern holographic displays is due toefforts to compensate for these deficiencies by, for example, adding eyetracking to deal with low diffraction angle [Haussler, R., Schwerdtner,A., and Leister, N., “Large holographic displays as an alternative tostereoscopic displays,” Proceedings of SPIE Stereoscopic Displays andApplications XIX, p. 68030M (2008)], duplicating and phase shifting theoptical path in order to eliminate the zero order [Chen, G.-L., Lin,C.-Y., Kuo, M.-K., and Chang, C.-C., “Numerical suppression ofzero-order image in digital holography.” Optics Express, vol. 15, pp.8851-8856 (2007)], or creating large arrays of spatial light modulatorsin order to increase the display size [Sato, K., A. Sugita, M. Morimoto,and K. Fujii, “Reconstruction of Color Images at High Quality by aHolographic Display”, Proc. SPIE Practical Holography XX, p. 6136(2006)]. The cost and complexity of holographic video displays can begreatly reduced if a spatial light modulator can be made to have betteraffordances than the LC and MEMS devices that are currently employed.

Full-color, video-rate holographic stereograms using arrays ofwaveguides with acoustic grating patterns that diffract in one axis only(horizontal parallax only or HPO) have previously been produced [D.Smalley, Q. Smithwick, V. M. Bove, Jr., J. Barabas, S. Jolly,“Anisotropic leaky-mode modulator for holographic video displays.”Nature 498.7454, pp. 313-317 (2013); D. Smalley, Q. Smithwick, J.Barabas, V. M. Bove, Jr., S. Jolly, and C DellaSilva, “Holovideo foreveryone: a low-cost holovideo monitor,” Proc. 9th InternationalSymposium on Display Holography (ISDH 2012) (2012)]. The advantages ofpolarization rotation, enlarged angular diffraction, and wavelengthdivision for red, green, and blue light have therefore beendemonstrated.

SUMMARY

In one aspect, the present invention is a transparent holographic videodisplay system that is suitable for near-to-eye augmented reality andsee-through applications. Based on monolithic guided-wave acousto-opticswith integrated volume gratings, a preferred embodiment has a compositedisplay comprised of space-multiplexed elemental modulators, each ofwhich exploit leaky-mode diffraction of guided-mode light. Eachelemental modulator consists of a surface acoustic wave transducer atopan anisotropic waveguide. In a typical implementation. each “line” ofthe overall display consists of a single anisotropic waveguide acrossthe display's length, with multiple surface acoustic wave transducersspaced along the length of the waveguide. For larger displays, thewaveguide may be divided into segments, with each being provided withseparate illumination. Light that is undiffracted by a specifictransducer is available for diffraction by subsequent transducers. Pertransducer, guided-mode light is mode-converted to leaky-mode light,which propagates into the substrate away from the viewer beforeencountering a volume reflection grating and being reflected and steeredtowards the viewer. The display is transparent and all reflection volumegratings operate in the Bragg regime, thereby creating no dispersion ofambient light.

In one aspect of the invention, a holographic video display comprises aplurality of space-multiplexed elemental modulators. Each elementalmodulator is configured to employ leaky-mode diffraction of guided-modelight to produce a line of a holographic display and includes ananisotropic waveguide, at least one in-coupling reflection gratingpositioned on the anisotropic waveguide at a location suitable forcoupling incident light into the waveguide to produce guided-mode lighttravelling in the waveguide, at least one surface acoustic wavetransducer disposed along the top of the anisotropic waveguide, whereineach surface acoustic wave transducer is configured to diffract theguided-mode light travelling in the waveguide into leaky-mode light, andat least one volume reflection grating positioned on the anisotropicwaveguide, each volume reflection grating being positioned at a locationsuitable for steering the leaky-mode light towards a viewer.

The display may include an electrical control layer comprising agraphics processing unit, circuitry for RF up-conversion andamplification, and a multiplexor for switching amongst holographic linesto drive multiple holographic lines in sequence. It include a substrateon which the plurality of elemental modulators are disposed. Thesubstrate may be lithium niobate. Each waveguide may be divided intosegments, each provided with separate illumination. The display may betransparent and all reflection volume gratings may operate in the Braggregime. Each waveguide may be associated with multiple one-to-oneassociated acoustic transducers and volume reflection gratings, arrangedalong the anisotropic waveguide to produce multiple output lines. Theremay be multiple acoustic transducers disposed along the anisotropicwaveguide in order to provide a desired length of optical line. Theinvention includes any holographic video image created by the display.

In another aspect of the invention, a method for generating aholographic image includes providing one or more wavelengths of light toa holographic video display, the display comprising a plurality ofspace-multiplexed elemental modulators, providing holographicinformation to the video display; coupling the light received at theholographic video display into the elemental modulators for diffractionaccording to the holographic information; and scanning the steered lightto form the holographic image. Each elemental modulator is configured toemploy leaky-mode diffraction of guided-mode light to produce a line ofa holographic display and includes an anisotropic waveguide, at leastone in-coupling reflection grating positioned on the anisotropicwaveguide at a location suitable for coupling incident light into thewaveguide to produce guided-mode light travelling in the waveguide, atleast one surface acoustic wave transducer disposed along the top of theanisotropic waveguide, wherein each surface acoustic wave transducer isconfigured to diffract the guided-mode light travelling in the waveguideinto leaky-mode light, and at least one volume reflection gratingpositioned on the anisotropic waveguide, each volume reflection gratingbeing positioned at a location suitable for steering the leaky-modelight towards a viewer. The invention includes a holographic videodisplay that employs the method.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, advantages and novel features of the invention willbecome more apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,wherein:

FIG. 1 is an x-y cross-section (side view) of an example guided opticalwave SAW device with integrated Bragg gratings, according to one aspectof the invention.

FIG. 2 is a z-y cross-section (top view) of the guided optical wave SAWdevice of FIG. 1, according to one aspect of the invention.

FIG. 3 depicts diffraction efficiency for red, green, and bluewavelengths vs. angular deviation from the Bragg angle for areflection-mode volume holographic Bragg grating with Λ=1 μm andthickness of 800 μm as indicated by Kogelnik's coupled-mode theory.

FIG. 4 depicts conservation of momentum in a nearly collinearacousto-optic guided-to-leaky mode transition.

FIG. 5 is an x-y cross-section (side view) of an example multi-elementSAW device, according to one aspect of the invention.

FIG. 6 is a z-y cross-section (top view) of the multi-element SAW Deviceof FIG. 5, according to one aspect of the invention.

FIG. 7 is an example timing diagram for pulsed laser illumination of SAWdevices.

FIG. 8 is an example timing diagram for hsync pulses, with τ_(fill)being the time duration over which the aperture is filled by theacoustic pixel stream.

FIG. 9 is a z-y cross-section (top view) of an example multi-element,multi-channel SAW device, according to one aspect of the invention.

FIG. 10 depicts an example embodiment of an electrical control layerpath for GPU signal output, RF up-conversion and amplification, andswitching amongst holographic lines for driving multiple holographiclines in sequence, according to one aspect of the invention.

FIG. 11 depicts focused laser spot in femtosecond laser micromachining.

FIGS. 12A-C depict index ellipsoids for uniaxial lithium niobate,wherein FIG. 12A depicts unperturbed LiNbO₃, FIG. 12B depictsproton-exchanged LiNbO₃, and FIG. 12C depicts femtosecond laserphoto-perturbed LiNbO₃.

DETAILED DESCRIPTION

A flat-panel, transparent holographic display solution suitable fornear-to-eye augmented reality applications according to the invention isbased on monolithic guided-wave acousto-optics with integrated volumegratings. A preferred embodiment has a composite display comprised ofspace-multiplexed elemental modulators, each of which exploit leaky-modediffraction of guided-mode light. Fabrication of modulatorsub-components may be achieved via femtosecond laser micromachiningprocesses.

The display is comprised of space-multiplexed elemental modulators, eachof which consists of a surface acoustic wave transducer atop ananisotropic waveguide. Each “line” of the overall display typicallyconsists of a single anisotropic waveguide across the display's lengthwith multiple surface acoustic wave transducers spaced along its length,although for larger displays, the waveguide may be divided intosegments, each provided with separate illumination. Light that isundiffracted by a specific transducer is available for diffraction bysubsequent transducers. Per transducer, guided-mode light ismode-converted to leaky-mode light, which propagates into the substrateaway from the viewer before encountering a volume reflection grating andbeing reflected and steered towards the viewer. The display istransparent and all reflection volume gratings operate in the Braggregime, thereby creating no dispersion of ambient light.

The present invention builds on and employs the guided-wave lightmodulation technologies previously described in U.S. patent applicationSer. No. 14/985,453, U.S. Pat. No. 8,149,265, U.S. patent applicationSer. No. 13/437,850, U.S. patent application Ser. No. 14/213,333, andU.S. patent application Ser. No. 14/217,215, all of which areincorporated by reference herein in their entirety. As a method fortransparent display, it allows for augmented reality applicationsnaturally. The use of integrated reflection volume gratings to directleaky-mode diffracted light towards a viewer presents an extremelylight- efficient solution for direct viewing of the displayed 3-Dwavefield from the surface of the modulator without any additionalrequisite supporting optics. As a flat, transparent holographic display,the solution has natural applications in augmented reality but can alsobe adapted for virtual reality. Larger versions of the device could beused in heads-up displays, see-through hand-held devices, and similarapplications.

A preferred embodiment of a system according to the invention employs aguided-wave acousto-optic platform implemented in lithium niobate(LiNbO₃), in order to realize a fully-monolithic, transparent,flat-panel holographic video display.

Basic Optical Principles. The optical design employs several conceptsthat have already been explored in other contexts: (1) the use ofanisotropic guided-wave acousto-optics for spatial light modulation inholographic video displays [D. E. Smalley, Holovideo on a Stick:Integrated Optics for Holographic Video Displays, Ph. D. Thesis,Massachusetts Institute of Technology, 2013; D. E. Smalley, Q. Y. J.Smithwick, V. M. Bove, Jr., J. Barabas and S. Jolly, “Anisotropicleaky-mode modulator for holographic video displays,” Nature, v. 498,pp. 313-317, 2013] via a guided-to-leaky mode transition in birefringentLiNbO₃ [D. V. Petrov and J. Ctyroky, “Acousto-optic conversion of aguided mode into a leaky wave in a Ti:LiNbO3 waveguide,” Pis'ma vZhurnal Tekhnicheskoi Fiziki, vol. 9, pp. 1120-1124, September 1983; A.M. Matteo, C. S. Tsai, and N. Do, “Collinear guided wave to leaky waveacoustooptic interactions in proton-exchanged LiNbO₃ waveguides,” IEEETrans. Ultrason., Ferroelect., Freq. Contr., vol. 47, no. 1, pp. 16-28],(2) the use of a Bragg grating to introduce illumination into awaveguide [C. S. Tsai, Guided-Wave Acousto-Optics: Interactions Devicesand Applications. Springer-Verlag, 1990], (3) the use of beam strobingin order to “freeze” the acousto-optic pattern and eliminate the needfor de-scanning the propagating acoustic wave [W. Akemann, J.-F. Lager,C. Ventalon, B. Mathieu, S. Dieudonné, and L. Bourdieu, “Fast spatialbeam shaping by acousto-optic diffraction for 3D non-linear microscopy,”Optics Express, vol. 23, no. 22, pp. 28191-28205, November 2015], and(4) the use of a volume holographic Bragg reflection grating in order toreflect the leaky diffracted toward a viewer with high efficiency [H.Kogelnik, “Coupled wave theory for thick hologram gratings,” The BellSystem Technical Journal, 1969; J. Hukriede, D. Runde, and D. Kip,“Fabrication and application of holographic Bragg gratings in lithiumniobate channel waveguides,” J. Phys. D: Appl. Phys., vol. 36, no. 3,pp. R1-R16, February 2003]. All elements, including the in-couplinggrating, anisotropic waveguide, and output volume hologram, can berealized within a single LiNbO₃ substrate without the need for anyadditional supporting optics. The platform therefore provides a pathtowards a fully-monolithic, integrated-optic platform for transparentholographic video display for near-to-eye display and other see-throughdisplay applications.

FIGS. 1 and 2 depict the basic structure of an example guided opticalwave surface acoustic device according to a preferred embodiment of theinvention. For x-cut LiNbO₃, the z-axis is the extraordinary axis.

FIG. 1 is an x-y cross-section (side view) of a guided optical wave SAWdevice with integrated Bragg gratings, according to one aspect of theinvention. In the device of FIG. 1, the incident light 105 is linearlypolarized in the TE orientation and is reflected off surface (TE) Braggin-coupling reflection grating 110, which couples it into anisotropic(TE) waveguide 115 (n_(e)>n_(e0)) on bulk X-cut lithium niobate (n_(e0),n_(o0)) substrate 120, producing guided-mode light 125. The waveguide115 has an extraordinary index perturbation of Δn_(e) relative to thesubstrate 120, but no ordinary index change. When excited by an RFsignal containing the holographic information, the interdigitatedelectrodes 130 (pictured in FIG. 2) launch a surface acoustic wave (SAW)135. The guided-mode light 125 interacts with the SAW 135 and ismode-converted into a diffracted TM mode that exits the waveguide as aleaky mode 140. Upon entering the substrate region 120, the leaky mode140 is incident upon a reflection mode volume holographic Bragg grating145 (Δn_(e), Δn_(o)) with grating vector nearly parallel to the centerwavevector of the incident leaky mode's angular fan. Due to the volumehologram's wide angular acceptance range (see FIG. 3), the leaky mode isreflected 150 to exit substrate 120 through the waveguide 115 towardsthe viewer 155.

FIG. 2 is a z-y cross-section (top view) of the example SAW device ofFIG. 1. For x-cut LiNbO₃, the z-axis is the extraordinary axis. In FIG.2, β_(guided) 210 is the propagation constant of the guided-mode TElight 125 (FIG. 1) in the waveguide 115 and {right arrow over(K)}_(grating) 220 is the acoustic grating's wavevector. Thesequantities obey the conservation relationship depicted in FIG. 4.

Established frequency-division mutliplexing schemes for full-coloroperation [D. E. Smalley, Holovideo on a Stick: Integrated Optics forHolographic Video Displays, Ph. D. Thesis, Massachusetts Institute ofTechnology, 2013; D. E. Smalley, Q. Y. J. Smithwick, V. M. Bove, Jr., J.Barabas and S. Jolly, “Anisotropic leaky-mode modulator for holographicvideo displays,” Nature, v. 498, pp. 313-317, 2013] can similarly beapplied to the device. The device can also be operated for use in ascanning retinal display [B. T. Schowengerdt and E. J. Seibel,“Stereoscopic retinal scanning laser display with integrated focus cuesfor ocular accommodation,” Proceedings of SPIE/IS&T StereoscopicDisplays and Virtual Reality Systems XI, vol. 5291, 2004].

FIG. 3 depicts the calculated diffraction efficiency for red 310, green320, and blue 340 wavelengths vs. angular deviation from the Bragg anglefor a reflection-mode volume holographic Bragg grating with Λ=1 μm andthickness of 800 μm, as indicated by Kogelnik's coupled-mode theory [H.Kogelnik, “Coupled wave theory for thick hologram gratings,” The BellSystem Technical Journal, 1969; I. V. Ciapurin, L. B. Glebov, and V. I.Smirnov, “Modeling of Gaussian beam diffraction on volume Bragg gratingsin PTR glass,” Proceedings of SPIE Practical Holography XIX: Materialsand Applications, vol. 5742, pp. 183-194, April 2005]. The wide angularacceptance range allows for the total angular extent of a leaky mode tobe reflected with high efficiency. Furthermore, wavelength mutliplexingof several Bragg holograms can enable full-color operation [G.Barbastathis and D. Psaltis, “Volume holographic multiplexing methods inHolographic Data Storage, Eds: H. Coufal, D. Psaltis, and G. Sincerbox.Springer, N.Y., 2000].

FIG. 4 depicts conservation of momentum (phase-matching condition) in anearly collinear acousto-optic guided-to-leaky mode transition.β_(guided) is the propagation vector of the TE-polarized guided modelight (i.e., the component of the guided mode wavevector along thepropagation direction), β_(leaky) is the component of the TM-polarizedleaky mode light along the waveguide axis, {right arrow over(K)}_(grating) is the acoustic grating wavevector, and {right arrow over(K)}_(leaky) is the total wavevector of the leaky mode light.

The device pictured in FIGS. 1 and 2 represents a single acousto-opticelement capable of modulating only some portion of a holographic image(i.e., in an elemental hologram sense). This is due to the fact that thesurface acoustic wave has only a limited interaction length with theguided-mode light before the efficiency of the interaction approacheszero. Therefore, placement of several acousto-optic transducers on thesame waveguide is necessary in order to obtain a longer holographicline.

Structurally, a multi-element device is comprised of multiple elementsof the type depicted in FIG. 1. Guided-mode light that is undiffractedby a surface acoustic wave continues to propagate in the waveguide andis available for diffraction for subsequent surface acoustic waves.Multiple SAW transducers are positioned along the waveguide axis andinteract progressively with guided-wave light in a resonant fashion.Volume holographic Bragg gratings are positioned for reflection of everyleaky mode exiting the waveguide. This type of scheme is depicted inFIGS. 5 (side view) and 6 (top view).

FIG. 5 is an x-y cross-section (side view) of an example multi-elementSAW device, according to one aspect of the invention. As shown in FIG.5, incident TE light 505 is reflected off surface (TE) Bragg in-couplingreflection grating 510 (Δn_(e)), which couples it into anisotropic (TE)waveguide 515 (n_(e)>n_(e0)) on bulk X-cut lithium niobate (n_(e0),n_(o0)) substrate 520, producing guided-mode light 525. When excited byan RF signal containing the holographic information, the interdigitatedelectrodes 530, 531, 532 (pictured in FIG. 6) launch surface acousticwaves 535, 536, 537. The guided-mode light 525 interacts with the SAWs535, 536, 537 and is mode-converted into diffracted TM modes that exitsthe waveguide as leaky modes 540 541, 542. Light 525 that isundiffracted by the first transducer 530 is available for diffraction bysubsequent transducer 531, and so on. Upon entering the substrate region520, leaky modes 540, 541, 542 are incident upon respective reflectionmode volume holographic Bragg gratings 545, 546, 547 (Δ_(e), Δn_(o))with grating vectors nearly parallel to the center wavevector of theincident leaky mode's angular fan. The leaky modes are reflected 550,551, 552 by respective reflection mode volume holographic Bragg gratings545, 546, 547 to exit the substrate 520 through the waveguide 515towards the viewer.

FIG. 6 is a z-y cross-section (top view) of the multi-element SAW Deviceof FIG. 5, according to one aspect of the invention. In FIG. 6,β_(guided) 610 is the propagation constant of the guided-mode TE light525 (FIG. 5) in waveguide 515 and {right arrow over (K)}_(grating) 620is the acoustic grating's wavevector.

Systems Engineering. Strobed (pulsed laser illumination) operation hasbeen presented as a solution to overcoming non-stationarity inacousto-optic modulators when used for beam shaping applications [W.Akemann, J.-F. Lager, C. Ventalon, B. Mathieu, S. Dieudonné, and L.Bourdieu, “Fast spatial beam shaping by acousto-optic diffraction for 3Dnon-linear microscopy,” Optics Express, vol. 23, no. 22, pp.28191-28205, November 2015]. Such a scheme can be used in place ofpolygonal mirror scanning techniques that have been applied previouslyfor scanned-aperture holographic video displays based aroundacousto-optic modulators [D. E. Smalley, Holovideo on a Stick:Integrated Optics for Holographic Video Displays, Ph. D. Thesis,Massachusetts Institute of Technology, 2013; D. E. Smalley, Q. Y. J.Smithwick, V. M. Bove, Jr., J. Barabas and S. Jolly, “Anisotropicleaky-mode modulator for holographic video displays,” Nature, v. 498,pp. 313-317, 2013; P. S. Hilaire, S. A. Benton, and M. Lucente,“Synthetic aperture holography: a novel approach to three-dimensionaldisplays,” Journal of the Optical Society of America A, vol. 9, no. 11,pp. 1969-1977, 1992; P. St. Hilaire, Scalable Optical Architectures forElectronic Holography, Ph. D. Thesis, Massachusetts Institute ofTechnology, 1994].

An example timing diagram for strobed operation of a device according tothe invention is depicted in FIG. 7. In FIG. 7, τ_(fill) 710 is the timeduration over which the aperture is filled by the acoustic pixel streamand τ_(pixel) 720 is the time duration over which a single pixel isacoustically drawn. The duty cycle is then D=τ_(pixel)/τ_(fill).τ_(fill) can be found as τ_(fill)=l/v, where l is the interaction lengthand v is the velocity of the propagating surface acoustic wave. Forx-cut LiNbO₃, v=3909 m/s; assuming an interaction length l=1 cm,τ_(fill)=2.558 μs. For a 400 Mpixel/s pixel clock from a modern graphicsprocessing unit, τ_(pixel)= 1/400 Hz=2.5 ns. Each illumination pulse istied to the length of time taken for the graphics processing unit tooutput one filled aperture's worth of pixels; this can readily be set tobe one horizontal line on the GPU framebuffer and hence the pulses canbe triggered on the GPU's hsync pulses (depicted in FIG. 8).

Where the GPU or other video generation circuitry does not support asufficiently long line length for the necessary diffraction pattern, theaperture may be spread across multiple framebuffer lines and theillumination triggered by a counter driven by hsync pulses. FIG. 8 is anexample timing diagram for hsync pulses, with τ_(fill) 810 being thetime duration over which the aperture is filled by the acoustic pixelstream.

Each waveguide, being driven with either a single or multiple SAWtransducers, is responsible for delivering a single horizontalparallax-only holographic line to the viewer. In order to deliverimagery with greater vertical resolution, multiple such holographiclines are required in the output. Scanned-aperture displays based aroundbulk-wave acousto-optic modulators [P. S. Hilaire, S. A. Benton, and M.Lucente, “Synthetic aperture holography: a novel approach tothree-dimensional displays,” Journal of the Optical Society of AmericaA, vol. 9, no. 11, pp. 1969-1977, 1992; P. St. Hilaire, Scalable OpticalArchitectures for Electronic Holography, Ph. D. Thesis, MassachusettsInstitute of Technology, 1994] or guided-wave acousto-optic devices [D.E. Smalley, Holovideo on a Stick: Integrated Optics for HolographicVideo Displays, Ph. D. Thesis, Massachusetts Institute of Technology,2013; D. E. Smalley, Q. Y. J. Smithwick, V. M. Bove, Jr., J. Barabas andS. Jolly, “Anisotropic leaky-mode modulator for holographic videodisplays,” Nature, v. 498, pp. 313-317, 2013] employ scanninggalvanometers to optically scan multiple holographic lines within thepersistence time of the human eye. However, a flat-panel holographicvideo display requires that no supporting optics be used. Therefore,increased vertical resolution can only be achieved via the use ofadditional waveguide channels [D. E. Smalley, Holovideo on a Stick:Integrated Optics for Holographic Video Displays, Ph. D. Thesis,Massachusetts Institute of Technology, 2013].

Such a scheme is depicted in FIG. 9, which depicts a z-y cross-section(top view) of an example multi-element, multi-channel SAW device,according to this aspect of the invention. In this scheme, light iscoupled into all waveguides simultaneously via in-coupling Bragggratings 910, 911, 912 that are positioned on top of each waveguide 915,916, 917 on substrate 920. As in the example device shown in FIGS. 5 and6, multiple SAW transducers 930, 931, 932, 933, 934, 935, 936, 937, 938and reflection Bragg gratings 910, 911, 912 are positioned along thelength of each waveguide 915, 916, 917.

In order to electrically drive the entire example multi-element,multi-channel device depicted in FIG. 9 with holographic informationwith the limited temporal bandwidth available from modern GPUs, eachcolumn of SAW elements spanning multiple waveguides may be driven by asingle analog output channel of a graphics processing unit in atime-sequential fashion. This allows for the possibility of coherenceamongst surface acoustic waves generated by multiple SAW transducerelements on a single waveguide, so that all of the holographicinformation spanning multiple SAW transducers on a single holographicline is drawn at the same time, as well as reduces the number of analogGPU channels needed.

Such a scheme can be implemented via the use of an analog RFdemultiplexer, as shown in FIG. 10. FIG. 10 depicts an exampleembodiment of an electrical path for GPU signal output, RF up-conversionand amplification, and switching amongst holographic lines for drivingmultiple holographic lines in sequence, according to one aspect of theinvention. In FIG. 10, the output analog video signal 1005 from graphicscard (GPU) 1010 (containing holographic information) is appropriatelyup-converted via RF mixer 1015 with local oscillator 1020 to the RFfrequency operating band of SAW transducers 1025, 1026, 1027, 1028, 1030and amplified by RF amplifier 1035. This up-converted, amplified signal1040 is input to 1-to-2^(N) RF de-multiplexer 1045, which acts to switchinput signal 1040 to one of 2^(N) outputs 1050, 1051, 1052, 1053, 1055depending on control input 1060. Because input signal 1040 should beswitched based on the index of the current holographic line beingwritten, control input 1060 is the output of counter 1065 thataccumulates the number of hsync pulses output 1070 from the GPU 1010.This control scheme necessitates that the holographic informationdriving a single transducer is contained on a single framebuffer line inthe GPU's memory. During the duration between hsync pulses, theholographic information for the ith holographic line is drawn. Afterdrawing is completed, the GPU 1010 fires an hsync pulse 1070,incrementing the pulse counter 1065, and thereby switching the output toa transducer on the next holographic line. After all lines have beenswitched to and drawn, counter 1065 is reset 1075 upon receiving a vsyncpulse 1080 from GPU 1010 and de-multiplexer 1045 is thereby reset tooutput 1050 to the first holographic line's transducer 1025.

All independent sequential transducers per holographic line may beaddressed by independent GPU channels and de-multiplexing hardware. Inthis way, multiple transducers per holographic line are addressed in aparallel fashion, while separate holographic lines are addressed in aserial fashion. Other variations on this addressing scheme, as would beknown to those skilled in the art, may be used as appropriate for thenumber of lines, transducers, and simultaneous video signals available.

Fabrication via Femtosecond Laser Micromachining. Femtosecond lasermicromachining has emerged in the last several decades as a versatiletool for the fabrication of microdevices in transparent materials [R. R.Gattass and E. Mazur, “Femtosecond laser micromachining in transparentmaterials,” Nature Photonics, vol. 2, no. 4, pp. 219-225, April 2008].The use of femtosecond laser micromachining has been explored for thegeneration of waveguides [M. Dubov, S. Boscolo, and D. J. Webb,“Microstructured waveguides in z-cut LiNbO₃ by high-repetition ratedirect femtosecond laser inscription,” Optical Materials Express, vol.4, no. 8, pp. 1708-1716, August 2014; R. He, Q. An, Y. Jia, G. R.Castillo-Vega, J. R. V. de Aldana, and F. Chen, “Femtosecond lasermicromachining of lithium niobate depressed cladding waveguides,”Optical Materials Express, vol. 3, no. 9, pp. 1378-1384, September 2013;J. Burghoff, H. Hartung, S. Nolte, and A. Tünnermann, “Structuralproperties of femtosecond laser-induced modifications in LiNbO3,”Applied Physics A, vol. 86, no. 2, pp. 165-170, 2007; J. Burghoff, S.Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecondlaser-structured LiNbO3,” Applied Physics A, vol. 89, no. 1, pp.127-132, 2007], surface gratings [D. Grando, J. Yu, D. Ballarini, and P.Galinetto, “Femtosecond Laser Writing of Surface Microstructures inLithium Niobate,” Nonlinear Guided Waves and Their Applications (2005),paper WD33, p. WD33, September 2005], Bragg volume gratings [V.Mizeikis, V. Purlys, D. Paipulas, and R. Buividas, “Direct LaserWriting: Versatile Tool for Microfabrication of Lithium Niobate,”Journal of Laser Micro/Nanomachining, 2012; D. Paipulas, V. Kudria{hacekover (s)}ov, M. Malinauskas, V. Smilgevi{hacek over (c)}ius, and V.Sirutkaitis, “Diffraction grating fabrication in lithium niobate and KDPcrystals with femtosecond laser pulses,” Applied Physics A, vol. 104,no. 3, pp. 769-773, 2011], and complex integrated optic devices [J.Thomas, M. Heinrich, P. Zeil, V. Hilbert, K. Rademaker, R. Riedel, S.Ringleb, C. Dubs, J. P. Ruske, S. Nolte, and A. Tünnermann, “Laserdirect writing: Enabling monolithic and hybrid integrated solutions onthe lithium niobate platform,” Physica Status Solidi (a), vol. 208, no.2, pp. 276-283, February 2011] in lithium niobate. Recently, the use offemtosecond laser micromachining has been proposed as an alternative toproton exchange for the creation of anisotropic waveguides in LiNbO₃ forspatial light modulators for holographic video devices [N. Savidis, S.Jolly, B. Datta, T. Karydis, and V. Michael Bove, Jr., “Fabrication ofwaveguide spatial light modulators via femtosecond lasermicromachining,” Proc. SPIE Advanced Fabrication Technologies forMicro/Nano Optics and Photonics IX, 9759, 2016].

While many fabrication methods for devices according to the inventionare known in the art and would be suitable, in a preferred embodiment, adevice according to the invention can be fabricated entirely via afemtosecond laser micromachining process. The anisotropic waveguide canbe fabricated by an index change Δn_(e), which has been shown to bepossible via short duration pulse widths [J. Burghoff, H. Hartung, S.Nolte, and A. Tünnermann, “Structural properties of femtosecondlaser-induced modifications in LiNbO3,” Applied Physics A, vol. 86, no.2, pp. 165-170, 2007], relative to the substrate. The Bragg in-couplinggrating can be fabricated via surface ablation [D. Grando, J. Yu, D.Ballarini, and P. Galinetto, “Femtosecond Laser Writing of SurfaceMicrostructures in Lithium Niobate,” Nonlinear Guided Waves and TheirApplications (2005), paper WD33, p. WD33, September 2005] or viarefractive index modulation Δn_(e) [D. Paipulas, V. Kudria{hacek over(s)}ov, M. Malinauskas, V. Smilgevi{hacek over (c)}ius, and V.Sirutkaitis, “Diffraction grating fabrication in lithium niobate and KDPcrystals with femtosecond laser pulses,” Applied Physics A, vol. 104,no. 3, pp. 769-773, 2011]. The out-coupling grating can be fabricatedvia isotropic refractive index modulation in the bulk of the substrate.Metal SAW transducers may also be fabricated [T. Gertus, P. Ka{hacekover (z)}dailis, R. Rimeika, D. Ciplys, and V. Smilgevi{hacek over(c)}ius, “Surface acoustic wave transducers fabricated by femtosecondlaser ablation”, Electronics Letters, vol. 46, no. 17, 19 August 2010].

FIG. 11 depicts an example focused laser spot in femtosecond lasermicromachining. Material perturbation or ablation only occurs within asmall region around the focus. Shown in FIG. 11 are glass material 1120,absorption spot 1130, and caustic 1140.

FIGS. 12A-C depict example index ellipsoids for uniaxial lithiumniobate, wherein FIG. 12A depicts unperturbed LiNbO₃, FIG. 12B depictsproton-exchanged LiNbO₃, and FIG. 12C depicts femtosecond laserphoto-perturbed LiNbO₃.

While the proton exchange process in LiNbO₃ increases the extraordinaryindex while decreasing the ordinary index (Δn˜10⁻²), femtosecond lasermicromachining can induce an increase in extraordinary index only(Δn_(e)˜10⁻³) (see FIG. 12) when creating so-called Type I waveguides[J. Burghoff, H. Hartung, S. Nolte, and A. Tünnermann, “Structuralproperties of femtosecond laser-induced modifications in LiNbO3,”Applied Physics A, vol. 86, no. 2, pp. 165-170, 2007; J. Burghoff, S.Nolte, and A. Tünnermann, “Origins of waveguiding in femtosecondlaser-structured LiNbO3,” Applied Physics A, vol. 89, no. 1, pp.127-132, 2007]. The effect of the waveguide's index profile on theguided-to-leaky mode conversion process is to be determined viasimulation (i.e., coupled-mode theories dictating propagation inanisotropic waveguides [D. Marcuse, “Coupled-mode theory for anisotropicoptical waveguides,” The Bell System Technical Journal, vol. 54, no. 6,pp. 985-995, 1975] and interaction of guided-mode light with surfaceacoustic waves [A. M. Matteo, C. S. Tsai, and N. Do, “Collinear guidedwave to leaky wave acoustooptic interactions in proton-exchanged LiNbO₃waveguides,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 47,no. 1, pp. 16-28]).

Other recent advances in fabrication using femtosecond lasermicromachining may be suitable, such as femtosecond laser-baseddirect-write approaches for the fabrication of waveguide in-couplinggratings and volume Bragg reflection gratings via permanent refractiveindex changes within the lithium niobate substrate [Nickolaos Savidis,Sundeep Jolly, Bianca Datta, Michael Moebius, Thrasyvoulos Karydis, EricMazur, Neil Gershenfeld, and V. Michael Bove, Jr., “Progress infabrication of waveguide spatial light modulators via femtosecond lasermicromachining”, Proc. SPIE Advanced Fabrication Technologies forMicro/Nano Optics and Photonics X, 10115, 2017]. In combination withmetal surface-acoustic-wave transducers, these direct-write approachesallow for complete fabrication of a functional spatial light modulatorvia femtosecond laser direct writing.

Additionally, or alternatively, laser induced forward transfer (LIFT)[Bianca C. Datta, Nickolaos Savidis, Michael Moebius, Sundeep Jolly,Eric Mazur, and V. Michael Bove, Jr., “Direct-laser metal writing ofsurface acoustic wave transducers for integrated-optic spatial lightmodulators in lithium niobate”, Proc. SPIE Advanced FabricationTechnologies for Micro/Nano Optics and Photonics X, 10115, 2017] may beemployed for fabricating devices according to the invention. In thisprocess, metal is placed on an optically transparent substrate, which isthen placed against the target substrate. Specific patterns are writtenusing a high-precision three axis stage to move the substrates. DuringLIFT, the laser is used to ablatively transfer material from a thin filmon a support substrate to a target substrate by pulsed laser through aphotothermal deposition process via vapor-driven propulsion of metalfrom the film onto the second (target) substrate. As the substratematerial primarily experiences multi-photon effects (which are minimalhere), absorption of laser energy occurs primarily at themetal-substrate interface since the majority of energy is absorbed bythe metal, with laser light attenuation toward the surface of the metalfilm.

In addition to the foregoing, at least the following implementations,modifications, and applications of the described technology arecontemplated by the inventors and are considered to be within the scopeof the invention: pulsed illumination to create a stationary displayoutput in conjunction with the use of traveling acoustic waves for indexmodulation, use of integrated volume reflection gratings to directdiffracted leaky-mode light towards a viewer, and use of an RF switchingscheme in conjunction with an analog GPU output to allow fortime-multiplexed,“rastered” operation.

While preferred embodiments of the invention are disclosed herein, manyother implementations will occur to one of ordinary skill in the art andare all within the scope of the invention. Each of the variousembodiments described above may be combined with other describedembodiments in order to provide multiple features. Furthermore, whilethe foregoing describes a number of separate embodiments of theapparatus and method of the present invention, what has been describedherein is merely illustrative of the application of the principles ofthe present invention. Other arrangements, methods, modifications, andsubstitutions by one of ordinary skill in the art are therefore alsoconsidered to be within the scope of the present invention.

What is claimed is:
 1. A holographic video display comprising: aplurality of space-multiplexed elemental modulators, wherein eachelemental modulator is configured to employ leaky-mode diffraction ofguided-mode light to produce a line of a holographic display, eachelemental modulator comprising: an anisotropic waveguide; at least onein-coupling reflection grating positioned on the anisotropic waveguideat a location suitable for coupling incident light into the waveguide toproduce guided-mode light travelling in the waveguide; at least onesurface acoustic wave transducer disposed along the top of theanisotropic waveguide, each surface acoustic wave transducer configuredto diffract the guided-mode light travelling in the waveguide intoleaky-mode light; and at least one volume reflection grating positionedon the anisotropic waveguide, each volume reflection grating beingpositioned at a location suitable for steering the leaky-mode lighttowards a viewer.
 2. The holographic video display of claim 1, furthercomprising an electrical control layer, the electrical control layercomprising a graphics processing unit, circuitry for RF up-conversionand amplification, and a multiplexor for switching amongst holographiclines to drive multiple holographic lines in sequence.
 3. Theholographic video display of claim 1, further comprising a substrate onwhich the plurality of elemental modulators are disposed.
 4. Theholographic video display of claim 3, wherein the substrate is lithiumniobate.
 5. The holographic video display of claim 1, wherein eachanisotropic waveguide is divided into segments, each provided withseparate illumination.
 6. The holographic video display of claim 1,wherein the display is transparent and all reflection volume gratingsoperate in the Bragg regime.
 7. The holographic video display of claim1, wherein each anisotropic waveguide is associated with multipleone-to-one associated acoustic transducers and volume reflectiongratings, arranged along the anisotropic waveguide to produce multipleoutput lines.
 8. The holographic video display of claim 7, furthercomprising a substrate on which the plurality of elemental modulatorsare disposed.
 9. The holographic video display of claim 1, wherein thereare multiple acoustic transducers disposed along the anisotropicwaveguide in order to provide a desired length of optical line.
 10. Aholographic video image produced using the display of claim
 1. 11. Amethod for generating a holographic image, comprising: providing one ormore wavelengths of light to a holographic video display, the displaycomprising a plurality of space-multiplexed elemental modulators,wherein each elemental modulator is configured to employ leaky-modediffraction of guided-mode light to produce a line of a holographicdisplay, each elemental modulator comprising: an anisotropic waveguide;at least one in-coupling reflection grating positioned on theanisotropic waveguide at a location suitable for coupling incident lightinto the waveguide to produce guided-mode light travelling in thewaveguide; at least one surface acoustic wave transducer disposed alongthe top of the anisotropic waveguide, each surface acoustic wavetransducer configured to diffract the guided-mode light travelling inthe waveguide into leaky-mode light; and at least one volume reflectiongrating positioned on the anisotropic waveguide, each volume reflectiongrating being positioned at a location suitable for steering theleaky-mode light towards a viewer; providing holographic information tothe video display; coupling the light received at the holographic videodisplay into the elemental modulators for diffraction according to theholographic information; and scanning the steered light to form theholographic image.
 12. The method of claim 11, wherein the holographicvideo display further comprises an electrical control layer, theelectrical control layer comprising a graphics processing unit,circuitry for RF up-conversion and amplification, and a multiplexor forswitching amongst holographic lines to drive multiple holographic linesin sequence.
 13. The method of claim 11, wherein the holographic videodisplay further comprises a substrate on which the plurality ofelemental modulators are disposed.
 14. The method of claim 13, whereinthe substrate is lithium niobate.
 15. The method of claim 11, whereineach anisotropic waveguide is divided into segments, each provided withseparate illumination.
 16. The method of claim 11, wherein the displayis transparent and all reflection volume gratings operate in the Braggregime.
 17. The method of claim 11, wherein each anisotropic waveguideis associated with multiple one-to-one associated acoustic transducersand volume reflection gratings, arranged along the anisotropic waveguideto produce multiple output lines.
 18. The method of claim 17, whereinthe holographic video display further comprises a substrate on which theplurality of elemental modulators are disposed.
 19. The method of claim11, wherein there are multiple acoustic transducers disposed along theanisotropic waveguide in order to provide a desired length of opticalline.
 20. A holographic video display that employs the method of claim11.